It is known to provide a mechanism to vary the pitch of air-screws, such as in helicopters and air-planes. However, such mechanisms are highly complex and, therefore, expensive to manufacture and not suitable for windmills especially of the domestic type, which must be built at the least cost possible. In windmills, it is desirable to provide the air-screw with a variable pitch characteristic to prevent excessive speed of the air-screw in high wind condition and especially when the load on the air-screw is small or nil.
Air-screws, more particularly ventilator fans used in automobiles for cooling the liquid coolant in the radiator, are at the present time not equipped with a variable pitch feature; but it would be advantageous to so equip the same in order to automatically set the fan blades at zero pitch when the automobile travels at high speed, so as to conserve energy. Reverting to windmills, the mounting of the windmill on a support tower is generally rigid and the tower must therefore be firmly supported in an upright position to resist wind gusts. This considerably complicates the installation of windmills.